IN RECENT days there has been abundant evidence of Russia stepping up supplies of heavy weapons to rebels in eastern Ukraine, including advanced anti-aircraft systems. The Kiev government reported that two of its military aircraft were shot down in the past week, either by separatists, Russian planes or batteries operating from across the border. On Thursday came a greater tragedy: the destruction of a Malaysia Airlines passenger jet with 295 people aboard. Ukrainian authorities charged that it had been struck by a missile fired by a Russian-made surface-to-air battery supplied to Moscow’s Ukrainian proxies.

If the reports are confirmed, the Ukrainian separatists and their sponsors in Moscow will be responsible for a heinous crime. The United States and its allies must insist that those responsible be held accountable — including those in the Kremlin.

Predictably, the Ukrainian rebels were doing their best Thursday to cover up what occurred. They denied shooting down the plane, though the wreckage landed in territory they control near the Russian border. Their commander, a Russian citizen and former military officer named Igor Strelkov, claimed in a blog post published Thursday to have shot down a Ukrainian military transport plane, then removed the message. Russia’s state-controlled media joined in the obfuscation, casting blame on Ukrainian military forces and even suggesting the Malaysian plane was hit by a missile aimed at an aircraft carrying Russian President Vladi­mir Putin. For his part, Mr. Putin disingenuously claimed that Ukraine was at fault because it resumed military operations against the rebels — ignoring Russia’s supply of heavy arms to the insurgents.

The United States and its allies must not allow Russia and its agents to carry out a coverup. Instead they should insist on an international investigation, sponsored by the U.N. Security Council, to determine how the plane was downed and where the weapon used against it originated. The Putin government should be challenged to support such an investigation or be seen by the world to block it.

At the same time, Western governments should stop allowing special interests and their excessive caution to impede robust sanctions against Russia for its behavior in Ukraine. To its credit, the Obama administration announced new sanctions Wednesday afternoon. Targeting two Russian banks, two energy companies and a number of defense firms, they were the strongest yet taken. But the administration acted only after weeks of internal debate and negotiations with European governments that failed to produce comparable European Union action.

A man gestures at a crash site of a passenger plane near the village of Grabovo, Ukraine, Thursday, July 17, 2014. Ukraine said a passenger plane carrying 295 people was shot down Thursday as it flew over the country, and both the government and the pro-Russia separatists fighting in the region denied any responsibility for downing the plane. (Dmitry Lovetsky/AP)

The White House continues to avoid measures that could inflict crushing damage on the Russian financial system and force Mr. Putin and the elites around him to choose between aggression in Ukraine and Russia’s economic future. Wednesday’s measures stop two banks from obtaining long-term debt financing in U.S. markets, but the treasury has not excluded those banks from the financial system nor taken measures against the largest banks. Though its bank was sanctioned, the energy operations of Gazprom, Russia’s giant, remain untouched.

Even before the tragic loss of the Malaysian plane, it ought to have been clear to the West that Russia’s aggression in Ukraine could not be overlooked or wished away. It is both criminal and intolerable and should be treated accordingly.